Cable joints are the weakest part of power cable systems due to the presence of an interface between the cable insulation and the insulation of the accessory. Electrical tracking is one of the main failure mechanisms that can occur at dielectric interfaces. Therefore, this paper investigated the effect of interfacial smoothness on the tracking mechanism in XLPE-Si-Rubber interfaces. The interface was setup by slices of XLPE and silicon rubber in the experiment. The XLPE surfaces were polished into different smoothness by using abrasive papers. AC voltage was applied on a pair of flat-round electrodes at the interface with an insulation distance of 10 mm. The initial discharge voltage, the tracking failure voltage and the time to tracking failure with different interfacial smoothness were recorded and the tracking failure process at the interface was recorded with a high-speed camera. In order to quantify the discharge light and the carbonization degree, the photo patterns were analyzed with fractal dimension method. Obtained results showed that with the increase of interfacial smoothness, the initial discharge voltage, the tracking failure voltage and the time to tracking failure increased, but the fractal dimensions of the carbonization degree and the discharge light decreased.

Effect of surface smoothness on tracking mechanism in XLPE-Si-rubber interfaces

Interfaces between solid insulating materials are generally weak regions in electrical insulation systems, particularly if the electrical stress is applied in parallel direction. This paper presents a theoretical mechanical model for estimating average size of air-filled voids at the interface as a function of surface roughness and contact pressure. It is argued that the interfacial breakdown strength (BDS) is governed by the discharge inception stress (Ev) of the void with the most likely estimated dimension. The estimated values of the breakdown strength and partial discharge inception electric field strength (PDIE) were compared with the results from measurements using XLPE specimens with interfaces energized in longitudinal direction. The measured ratio of increase of breakdown strength was found to be in good agreement with the estimated breakdown values of the most likely interface's void. Additionally, it was found out that the air pressure inside voids was not affected by applied contact pressure. The estimated PDIE values was found to be in agreement with the measured values, in the case of rough surfaces, but not for smooth surfaces. The results indicate that the mechanical contact approach using the motif description of surface roughness may improve the understanding of the factors influencing the PDIE of electrically stressed interfaces.

Application of contact analysis on evaluation of breakdown strength and PD inception field strength of solid-solid interfaces

Interfaces between solids are generally considered weak regions in electrical insulation systems. This is particularly so if the electrical stress is applied parallel to the interface. Important parameters, affecting the breakdown strength, are interface pressure, humidity, presence of liquid dielectric and the surface roughness of the solid in contact. The main aim of the work presented here is to examine, theoretically and experimentally, the effect of interfacial pressure and roughness on the tangential breakdown strength. The size and gas pressure of enclosed surface voids were estimated using mechanical contact theory. The dielectric 50 Hz AC tangential strength of XLPE|XLPE interface was investigated for various values of pressure and roughness. The increase in breakdown strength due to increased pressure was largest in case of surfaces with the high degree of roughness. As expected the highest breakdown strength was observed in case of the smoothest surfaces. The estimated results of void size and gas pressure were found to be in good agreement with the experimental observations.

/pdf/breakdown-strength-of-solid-solid-interface-1mabr5ifol.pdf

Breakdown strength of solid|solid interface

Insulations enhanced with multilayer dielectrics are widely used in many applications. However, the interfaces between the dielectric layers are the weakest regions. Interfacial breakdown occurs frequently at the XLPE/SiR interfaces of DC cable joints or terminations. In this paper, an interface contact model is proposed to analyze the morphology of the XLPE/SiR interface, and the relationship between the interfacial ramped DC breakdown voltage and interface morphology is studied. Based on the interface contact model, the densities of real contact asperities and the percentages of the real contact area of XLPE/SiR interfaces under different levels of roughness and different interface pressures were calculated. Moreover, the interface contact model was verified by the results of an interface contact experiment. Interfacial DC breakdown tests were performed under different levels of roughness and different interface pressures. The interfacial ramped DC breakdown voltage increased from 20.37 to 36.05 kV with the grit number of the sandpaper used to sand the XLPE samples, i.e., 60, 150, 240, and 400. As the interface pressure increased from 0.1 to 0.5 MPa, the interfacial ramped DC breakdown voltage increased from 22.34 to 46.84 kV. The mechanism dictating the relationship between the interfacial ramped DC breakdown voltage and interface morphology was analyzed. The real contact area was considerably smaller than the nominal contact area, and many micro voids were present at the interfaces. These micro voids could be the main channel of interfacial breakdown, and the contact asperities could inhibit breakdown. A higher density and larger average area of real contact asperities results in more obstacles the breakdown encounters and causes the interfacial ramped DC breakdown voltage to be higher.

Relationship between the interfacial ramped DC breakdown voltage and the morphology of the XLPE/SiR interface

Double-layer dielectrics are widely used as insulation in electrical equipment, such as cable accessories. The interface between dielectrics is the weakest part of the insulation, which is critical to the overall insulation system. In this paper, the interface resistivity is studied as a parameter for evaluating the interface insulation performance. The interface resistivities of XLPE/SiR interfaces with different levels of roughness and pressures, as well as other interface types (XLPE/SiR, XLPE/EPDM, LDPE/SiR, LDPE/EPDM, HDPE/SiR, and HDPE/EPDM), were measured using an annular four-terminal electrode system. Moreover, interfacial ramped DC breakdown tests of XLPE/SiR interfaces with different levels of roughness and interface pressures were performed. As the roughness of the XLPE/SiR interface decreased, the interface resistivity increased from 5.10×1012 to 1.15×1013 Ω and the interfacial ramped DC breakdown voltage (U63.2%) increased from 20.48 to 36.80 kV. As the interface pressure of XLPE/SiR increased from 0.1 to 0.5 MPa, the interface resistivity increased from 9.72×1012 to 1.73×1013 Ω, and U63.2% increased from 26.49 to 53.61 kV. Interface resistivity is closely related to the contact morphology and surface resistivity, and an interface resistivity model is proposed to analyze the mechanism of the relationship between these factors. The interfacial ramped DC breakdown voltage has an obvious positive correlation with interface resistivity. Adopting interface resistivity as the characteristic parameter of interface insulation performance is beneficial for improving the reliability of interface insulation.

Interface resistivity and interfacial DC breakdown voltage of double-layer dielectrics

Great progress has been made in developing high-quality insulating materials, in maintaining consistent manufacturing processes, and in designing cable joints for easy installation. However, little attention has been given to the interfaces in cable joints. Greases are generally used as a lubricant for installation of premolded cable joints (excluding heat-shrink and cold-shrink splices), and are therefore always found at these interfaces. Analysis of field-aged cable joints has revealed two common problems related to greases: dry and sticky interfaces causing great difficulties in power disconnection, and interfacial discharges (or surface tracking) leading to dielectric breakdown. The focus of this study is on the dielectric performance of interfaces in cable joints. Measurements of breakdown voltage and pressure were carried out at interfaces of both new and field-aged cable joints. The effect of surface conditions was also investigated. Results show that the dielectric performance at interfaces is intimately related to the surface conditions, and a distinct long-term behavior is found for each type of interface in cable joints. The significance of the results and their implications are discussed.

D. Fournier

Papers

Dielectric Performance of Interfaces in Premolded Cable Joints